The mixing of hydrocarbon and oxygen-containing gases is required in many known processes. In many of these processes, the formed mixture may be in the flammable regime, by which is meant that the mixture, if it comes into contact with a suitable ignition source will combust. The flammability of fuel-oxygen mixtures, even within the flammable range can also vary. Thus, fuel-oxygen mixtures which are close to the stoichiometric composition possess higher burn rates than those mixtures found close to the flammable limits or are predominantly fuel rich. In particular, such mixtures are easily ignited and produce more stable flames that will rapidly propagate through a flammable mixture. These near stoichiometric mixtures can also lead to the generation of significant pressures and temperatures following ignition, and flames may only be extinguished by withdrawing a reactant.
It requires a finite time to mix fuel and oxygen and a wide range of mixtures may be formed during the mixing process. Thus, it is desirable to minimise the presence of more reactive, near stoichiometric, flammable mixtures even in mixtures where the nominal mixed composition is outside of the flammable regime or within the flammable range but with relatively low burn rate properties.
A number of other factors can increase the upper flammable limit, reducing the amount of oxygen required to sustain combustion, and expanding the range of potentially flammable gas mixtures, including, for example, the use of pure oxygen rather than air, increased temperature and pressure.
Rapid and efficient mixing is thus desired, and the object of an efficient mixer is to minimise the volume of highly reactive oxygen rich gas mixtures.
The auto-thermal cracking process is a known process for the production of olefins, in which a hydrocarbon and an oxygen-containing gas are mixed and subsequently contacted with a catalyst. The hydrocarbon is partially combusted on the catalyst, and the heat produced is used to drive the dehydrogenation of the hydrocarbon feed into olefins. An example of such a process is described in EP-A-0 332 289.
It is desired that the hydrocarbon and the oxygen-containing gas are uniformly mixed and preheated prior to contacting the catalyst. However, the mixture of hydrocarbon and oxygen-containing gas for the autothennal cracking process is flammable. Thus, it is desired to reduce the residence time in the reactor of the mixed gas stream, and in particular, it is usually desired, especially as pressure is increased, to mix the hydrocarbon and the oxygen-containing gas and contact them with the catalyst within significantly less than 1 second, usually significantly less than 100 ms.
WO 2004/074222 describes a reactor for the autothermal cracking of olefins, and in particular which provides a reactor design that enables an auto-thermal cracking process to be conducted wherein the gaseous reactants are preheated separately, mixed and subsequently contacted with a suitable catalyst. WO 2004/074222 describes two specific configurations for mixing first and second gaseous reactants (preferably hydrocarbon and oxygen-containing gases) in a uniform manner. One of these designs involves a tube-in-tube configuration with a plurality of conduits in each of which is provided an injection tube terminating in a nozzle through which the first gas exits into the second gas in the conduit.